1. Field of the Invention
The present invention relates generally to communication systems. Specifically, the invention relates to a method and apparatus for facilitating the reliable transfer of short, bursty messages over a wireless communications link.
2. Description of the Related Art
The use of wireless communication systems for the transmission of digital data is becoming more and more pervasive. In a wireless system, the most precious resource in terms of cost and availability is typically the wireless link itself. Therefore, one major design goal in designing a communication system comprising a wireless link is to efficiently use the available capacity of the wireless link. In addition, it is also desirable to reduce the delay associated with use of the link.
In a digital data system, remote units tend to generate bursty data to a hub station. The bursty data is characterized in that it has a high peak-to-average traffic ratio, meaning that blocks of data are transferred during short periods of time interposed between significantly longer periods of idleness.
In a time division multiple-access (TDMA) communication system, a separate time slot channel is assigned or dedicated to each remote unit. The remote unit uses the assigned time slot channel to transmit data to a hub station. By limiting transmissions to fall within the assigned time slot, the remote units are able to share the communication resources provided by the hub station. A TDMA system is effectively utilized when the transmission times and time slots of the remote units are all properly synchronized with each other.
In a TDMA system in which units have a pattern of use that includes bursty data, dedication of an individual time slot channel to each active remote unit does not result in efficient use of system capacity. This is because during those times when a remote unit is not utilizing the system, the time slot channel remains idle.
In a communication system with a plurality of remote units and a hub station, the hub station may detect when a remote unit has sent a burst to the hub station, determine the data content of the burst, and generate commands, such as timing synchronization signals, to feed back to the remote unit.
In present communication systems, the burst from a remote unit to the hub station typically includes a preamble which is used by the hub station to detect the burst. The added bits of the preamble increases the length of the burst and increases the duration of the time slot required by each remote unit. The more remote units in a system, the longer it takes the hub station to detect and process data bursts with their preambles. Furthermore, if the data bits in the burst are short, the preamble may be larger than the data bits. This results in an inefficient use of valuable system resources.
Thus, there is a need for a multiple-access system which provides advantageous use of system resources.
The present invention relates to a method and apparatus for facilitating the reliable transfer of short, bursty messages over a wireless communications link or channel. Specifically, the invention relates to a communication system in which a plurality of remote units encode data bursts using a predetermined codeword set and transmit the encoded data bursts to a hub station over a multiple-access channel. When the hub station receives a data burst from one of the remote units, the hub station resamples the data burst at a plurality of different timing offsets and/or carrier frequency offsets. The hub station correlates the received data burst with each codeword within the predetermined codeword set to determine which codeword has the maximum correlation. The hub station then correlates the codeword (with the maximum correlation) with the plurality of different timing and/or carrier frequency offset samples and derives a timing synchronization signal and/or a carrier frequency adjustment signal to be sent back to the remote unit. The signals provide information to the remote unit to synchronize its timing or adjust its carrier frequency for transmitting subsequent data bursts. The hub station may also use the results of the correlations to estimate a signal-to-noise ratio.
There are several features or advantages of the present invention. First, in one embodiment related to encoding the data bursts, a QPSK codeword set is advantageously created such that each codeword has the same distance configuration from neighboring codewords, i.e., each codeword has the same number of codewords at a given distance as all of the other codewords. This advantageously gives each codeword equal noise immunity. The entire codeword data set is also advantageously created to perform well in low signal-to-noise environments for low probability of transmission error.
Second, the remote units preferably encode data bursts with a non-coherent code. Use of a non-coherent code advantageously does not require the determination of a carrier phase associated with the encoded burst during demodulation (at the hub station).
Third, in one embodiment, each burst is relatively short and does not include a preamble. This reduces the amount of data sent via the channel and the resources required to code and decode any preamble bits. A short burst also allows the channel to accommodate a large number of frequent transmissions or opportunities to transmit via the channel from a large number of remote units.
Fourth, in one embodiment, the channel demodulation employs a correlation-based scheme that can advantageously estimate the signal in the presence of noise. This ability allows the channel to be less susceptible to corruption by noise and to operate in a relatively low signal-to-noise ratio environment.
Fifth, in one embodiment, the channel demodulation comprises a substantially uncomplicated correlation-based architecture. This feature facilitates a highly vectorized and pipelined implementation that may be executed rapidly on a current microprocessor-based system. Thus, no tradeoff in the speed of demodulation is required to optimally and accurately demodulate the signal received via the reservation channel.
Sixth, in one embodiment, the hub station derives a timing adjustment signal to be sent back to the remote unit after a burst or number of bursts have been received and processed by the hub station. There is preferably no adjustment to the transmission timing of the remote unit while the hub station is receiving and processing a burst from that particular remote unit. This reduces the time for the hub station to receive and process each burst. This allows a large number of remote units to access the channel and the hub station to process the messages received via the channel quickly and efficiently.
Seventh, in one embodiment, the hub station derives a carrier frequency adjustment signal to be sent back to the remote unit after a burst or number of bursts have been received and processed by the hub station. There is preferably no adjustment to the carrier frequency of the remote unit while the hub station is receiving and processing a burst from that particular remote unit. This reduces the time for the hub station to receive and process each burst. This allows a large number of remote units to access the channel and the hub station to process the messages received via the channel quickly and efficiently.
One aspect of the invention relates to a method of communicating in which a plurality of remote units transmit data to a hub station. The method comprises receiving a data burst from a remote unit where the data burst is encoded using a predetermined codeword set. The method further comprises resampling the data burst received by the hub station at a plurality of different timing offsets and correlating the data burst received by the hub station with the codeword set to find the codeword with the maximum correlation. The method further comprises correlating the codeword (with the maximum correlation) with the plurality of different timing offset samples and deriving a timing synchronization signal to be sent back to the remote unit. The timing synchronization signal provides information to the remote unit to synchronize its timing for transmitting subsequent data bursts.
Another aspect of the invention relates to a method similar to the one described above except the method includes deriving a carrier frequency adjustment signal, instead of or in addition to the timing synchronization signal, to be sent back to the remote unit. The carrier frequency adjustment signal provides information to the remote unit to adjust its carrier frequency for transmitting subsequent data bursts.
Another aspect of the invention relates to a method of communicating in a communication system in which a plurality of remote units transmit data to a hub station. The method comprises transmitting a data burst from a remote unit to the hub station over a non-contention, multiple-access channel. The method further comprises performing a plurality of complex correlations with the burst received by the hub station and estimating a signal-to-noise ratio of the non-contention access channel based on the complex correlations.
Another aspect of the invention relates to a method of accessing a system in which multiple remote units compete for limited communication resources. The method comprises transmitting a block of data over a contention-type access communication resource. The method further comprises encoding a corresponding notification message intended for the hub station using a non-coherent quadrature phase shift keying code, and transmitting the encoded notification message over a reserved communication resource.
Another aspect of the invention relates to a remote unit comprising an encoder adapted to encode a notification burst using a predetermined codeword set. The notification burst is configured to notify the hub station that the remote unit has sent a data burst to the hub station via a contention-type resource. The remote unit further comprises a transmitter adapted to transmit the encoded data burst from the remote unit to a hub station by using a non-contention, multiple-access channel.
Another aspect of the invention relates to a hub station comprising a receiver, at least one matched filter, a microprocessor and a timing synchronization circuit. The receiver is adapted to receive a data burst from a remote unit. The matched filter is adapted to resample the received data burst at a plurality of different timing offsets. The microprocessor is adapted to correlate the received data burst with a plurality of codewords from a codeword set to find a codeword with maximum correlation. The timing synchronization circuit is adapted to send a signal back to the remote unit, which provides information to the remote unit to synchronize its timing for transmitting subsequent data bursts.
One application of the present invention relates to encoding and decoding data associated with a reserved block or reservation channel in a communication system, which comprises three communication resources: a contention-type access block, a non-contention-type access block and a second non-contention access block called the reservation channel. Each time a remote unit has a block of data to transfer to a hub station, it sends the block of data over the contention-type access block. It also sends a corresponding notification message over the reservation channel. If the hub station receives the notification message but not the block of data, it sends a response message to the remote unit which designates a resource within the non-contention access block. The remote unit sends the block of data over the designated resource.